The objective of this study is to evaluate the accuracy of plasma iohexol clearance compared to the weekly creatinine and urea clearance in peritoneal dialysis patients. Iohexol (15 ml) will be injected slowly over 3-5 minutes intravenously. Clearance will be determined by the rate of disappearance of iohexol from the blood by X-ray fluorescence technique. Urine, dialysate and blood will also be measured for urea and creatinine clearance.